1. Field of the Invention
The present invention relates to an integrated optical device, for example a coupling device for coupling incident light into a waveguide. Specifically, the present invention relates to an optical coupling device and a method for manufacturing an optical coupling device.
2. Description of the Related Art
Light wave transmission can occur along optical fibres. Light can also be propagated through planar waveguide structures that can be implemented as photonic wave guides in integrated circuits. In many applications it is necessary to transmit an optical signal through a planar waveguide structure and to optically couple the light into or out of the integrated optical chip.
When testing optical circuitry on a chip it can be desirable to couple light from a core of an optical fibre, either perpendicularly or at an angle greater than zero degrees, into the integrated waveguide structure of the chip. This means that an efficient mechanism for coupling incident light into fabricated waveguides on a semiconductor wafer is desirable.
Not only is the coupling an issue but also the size mismatch of the light beam coming from, for example, a single mode fibre and the mode in the photonic waveguide implemented on a chip. The dimension of the integrated planar waveguide typically is much smaller than a light spot produced by an optical fibre.
In the past, cleaved facets on a semiconductor substrate surrounded by polymer tapers have been used to funnel the large light spot produced by a single mode optical fibre into the waveguide. Usually, the planar waveguide and the incident light beam are arranged in parallel. Such cleaved facet polymer tapers require large dimensions for delivering sufficient efficiency. However, the testing of such optical circuits on a wafer is difficult to achieve because of inconvenient lateral in-plane coupling.
Other conventional coupling mechanisms employ grating couplers with attached linear tapers. Hence, two separate devices are formed next to each other. A grating coupler comprises an array of parallel grating elements which are arranged on a substrate. Incident light, for example stemming from an optical fibre, is radiated perpendicularly or at an angle greater than zero degrees onto the grating plane. Through scattering, the light can than be coupled into the plane of the grating and fed into a photonic waveguide on the substrate.
However, the grated array is much larger than the diameter of the waveguide. Therefore, an adiabatic tapering employing an appropriate device is additionally used. Conventional gratings cover areas of around 10 by 10 μm wherein an adiabatic taper requires an additional length of about 500 μm until the modal size produced by the grating matches the modal size of the integrated waveguide of the respective optical chip. It is generally desirable to minimize the area assumed by such coupling and tapering devices.
U.S. Pat. No. 7,260,293 ('293) discloses an optical waveguide grating coupler that has a varying scatter cross section. This structure has bent gratings that correspond to curved wave fronts of light coming from an attached waveguide. In '293 two layer stacks are needed since the gratings are arranged on top of a funnel-shaped portion that merges into a corresponding waveguide.
In Michael M. Spühler, et al., “A very short planar silica spot-size converter using a nonperiodic segmented waveguide”, Journal of Lightwave Technology, vol. 16, No. 9, September 1998, page 1680 (Spühler), a segmented waveguide structure with an irregular tapering is introduced for laterally coupling light into a photonic wave guide. Spühler aims at integrating a spot-size converter with a waveguide. As mentioned above, lateral coupling may not be appropriate for testing a plurality of integrated optical circuits on a wafer.
Thus, it is desirable to design an improved optical coupling device.